Not Applicable
Not Applicable
1. Field of the Invention
This invention relates to a method of removing sulfur- and nitrogen-containing compounds from petroleum liquids and particularly to a method of desulfurization of fuel oils using aqueous acetic acid.
2. Description of Related Art
Environmental concerns have driven the need to remove many impurities from hydrocarbon based distillate fuels. Sulfur- and nitrogen-containing compounds are of particular interest because of their tendencies to produce precursors to acid rain and airborne particulate material. In addition, sulfur in particular can poison the catalysts used on automobiles and trucks to remove pollutant species. Several processes have been proposed in the past to deal with the problem of removing of these compounds from fuels. The most prevalent and common industrial process, and only large scale desulfurization process used to treat liquid fuels in refineries, is that of treating the fuel under high temperatures and high pressures with hydrogen. This process is called hydrotreating and has received extensive attention since its original invention in Germany before the Second World War. Literature describing this technology is immense, amounting to thousands of patents and scientific and engineering publications.
Briefly stated, hydrotreating is a process in which a petroleum fraction is heated, mixed with hydrogen, and fed to a reactor packed with a particulate catalyst. Temperatures in the reactor typically range from 600 to 700xc2x0 F. (315 to 370xc2x0 C.). At these temperatures, some or all of the feed may vaporize, depending on the boiling range of the feed and the pressure in the unit. For heavier feeds, it is common for the majority of the feed to be liquid. Reaction pressures range from as low as 500 psig (pounds per square inch, gauge) to as high as 2500 psig depending on the difficulty of removing the sulfur. In the manufacture of distillate fuels such as diesel or jet fuel, pressures higher than 800 psig are common. The feed and hydrogen mixture typically flows downward through the reactor, passing around and through the particulate catalyst. Upon leaving the reactor, the mixture of treated fuel and hydrogen flows through a series of mechanical devices to separate and recycle the hydrogen, remove poisonous hydrogen sulfide generated in the reaction, and recover the desulfurized product. Hydrotreating catalysts slowly lose activity with use, and must be removed and replaced every two to three years.
As used in large integrated refineries, hydrotreating is very effective, relatively inexpensive, and rather inefficient at removing the xe2x80x9crefractoryxe2x80x9d substituted benzo- and di-benzo-thiophenes. However, in small refineries, and especially those with limited capabilities, it can be prohibitively expensive because of the effects of scale-up economics. When process equipment is built, it typically costs much less than twice as much to build a unit with twice the capacity; engineers typically estimate that doubling the size increases the cost by only about 50%. The converse of the scale-up effect occurs when processes are scaled down; smaller process units are only slightly less expensive to build than larger ones. Thus the investment for a small 5,000 barrel per day (bpd) hydrotreater is about 25% of a 40,000 bpd hydrotreater and not 12.5% of the cost of the much larger unit; hence the unitary cost of the smaller unit is approximately twice that of the larger unit.
Because of the way processes are operated and controlled, the manpower costs for the smaller unit are roughly the same as those of the larger one.
Another cost problem faced by small refiners is the lack of an inexpensive hydrogen source. Hydrotreating typically consumes 200 to 500 scfb (standard cubic feet per barrel) of hydrogen, and may consume as much as 1000 scfb. Manufacture of hydrogen from natural gas typically costs about $3 per 1000 scf, adding about $0.60 to as much as $3.00 to the cost of treating a barrel of feed for a small refinery. In large refineries, hydrogen is often available as a byproduct of the gasoline manufacturing process known as platinum reforming. As such, it is virtually free. In small refineries with no platinum reformer, a dedicated hydrogen manufacturing plant must be installed, adding to the refinery operator""s investment burden and operating costs.
These economics favor those who wish to operate at large scale, but they make hydrotreaters prohibitively expensive for smaller refineries. This is one of the factors contributing to the closure of small refineries under the pressure of tightening environmental regulations. Some small refineries have survived by changing product mix to emphasize low value products such as asphalt, selling liquid products to large refineries to use as intermediates.
In order to continue to operate successfully, refineries and others have explored alternatives to hydrotreating. One idea that has been explored involves oxidizing the sulfur and nitrogen compounds in a distillate then removing them by selective extraction. This approach has met with only limited success primarily because of problems of non-selectivity of oxidants or the extraction solvents, problems that led to unacceptably high processing costs.
The complete removal of sulfur present in feedstock as sulfides, disulfides and mercaptans, is recognized as relatively easy, and comparatively inexpensive processes can accomplish this goal. Considerably more problematic are the family of xe2x80x9crefractory sulfur compounds.xe2x80x9d These compounds include the benzothiophenes and dibenzothiophenes and their mono-, di- and tri-substituted homologues with alkyl groups containing from one to 12 carbons. They are typically encountered within a boiling range of 220-350xc2x0 C., molar weight range of 134-300 Dalton, and carbon number of 8-24. These compounds have very high sulfur levels (in the range of 11-24 wt %). For example, the thiophenes found in Light Atmospheric Gas Oil (LAGO) from Alaska North Slope Crude containing about 5000 ppm sulfur typically have the following inspection:
In U.S. Pat. No. 3,847,800, Guth and Diaz proposed a process for treating diesel fuel that used oxides of nitrogen as the oxidant. However, nitrogen oxides have several disadvantages that can be traced to the mechanism by which they oxidize distillates. In the presence of oxygen, nitrogen oxides initiate a very non-selective form of oxidation termed auto-oxidation. Several side reactions also take place including the creation of nitro-aromatic compounds, oxides of alkanes and arylalkanes, and auto-oxidation products. Thus, nitrogen oxide based oxidants do not yield the appropriately oxidized sulfur compounds in distillate hydrocarbons without creating many undesirable byproducts.
The Guth and Diaz patent also proposes the use of methanol, ethanol, a combination of the two, and mixtures of these and water as an extraction solvent for polar molecules. Although these have proved to be acceptable extraction solvents for some polar compounds, they do not perform as well as others.
U.S. Pat. No. 4,746,420, issued to Darian and Sayed-Hamid also proposes the use of a nitrogen oxides to oxidize sulfur- and nitrogen-containing compounds followed by extraction using two solventsxe2x80x94a primary solvent followed by a cosolvent that is different from the primary. The sulfur and nitrogen results published in this patent are consistent with those expected from incomplete oxidation of these compounds followed by extraction.
In European Patent Application number 93302642.9 to Aida titled Method for Recovering Organic Sulfur Compounds from a Liquid Oil, Aida claims many oxidants as being essentially equal in their ability to oxidize sulfur- and nitrogen-containing compounds. However, it has been discovered that many of these oxidants are not selective and others are ineffective. Oxidizers that proceed by an auto oxidation mechanism involving a free radical tend not to be selective for the sulfur- and nitrogen-containing compounds of interest, producing numerous side reactions and, hence, various undesirable byproducts.
Aida teaches the use of distillation, solvent extraction, low temperature separation, adsorbent treatment and separation by washing to separate the oxidized organic sulfur compound from the liquid oil through the utilization of differences in the boiling point, melting point and/or solubility between the organic sulfur compound and the oxidized organic sulfur compound. While most of these work with some success, they do not provide the level of sulfur removal needed to meet environmental regulations.
In xe2x80x9cDesulfurization of Petroleum Fractions by Oxidation and Solvent Extractionxe2x80x9d, Fuel Processing Technology, 1995, 42, 35-45, by F. Zannikos, E. Lois, and S. Stournas, the authors describe an oxidation and solvent extraction technique for the removal of sulfur containing compounds. Peroxyacetic acid was used in an inefficient manner to oxidize the sulfur compounds in a diesel fuel. Methanol, dimethyl formamide, and N-methyl pyrrolidone were used as simple one-stage extraction solvents at different ratios. No mention of a process is made within this publication. Instead, the authors describe laboratory studies of the oxidation and extraction of sulfur compounds using methods like those taught in the art described above.
Two major problems are seen throughout this art. First, the oxidants chosen do not always perform optimally. Many oxidants engage in unwanted side reactions that reduce the quantity and quality of the treated fuels. The second problem is the selection of a suitable solvent for the extraction of the sulfur or nitrogen compounds. Using the non-optimum solvent may result in costly solvent recovery processing and removing desirable compounds from the fuel or extracting less than a desired amount of the sulfur and nitrogen compounds from the fuel. In either case, the results can be prohibitively expensive.
The reason for oxidizing the thiophenes in the feedstock to the corresponding sulfones or sulfoxides is to increase their polarity and molecular weight in order to facilitate their separation by extraction or distillation. The thermodynamics of the oxidation reaction is favorable, and it proceeds with reasonable selectivity at near-ambient temperature and pressure when the appropriate oxidant and operating conditions are selected. At least in theory, the final by-product can be elemental sulfur, sulfur dioxide or trioxide, sulfurous or sulfuric acid, or any of a variety of sulfur-containing salts. Most importantly, this approach avoids the need for using hydrogen and the attendant costs and safety issues. This technique is disclosed in the U.S. Pat. to Walter Gore, No. 6,160,193 entitled Method of Desulfurization of Hydrocarbons, which is incorporated herein by reference.
Reaction selectivity, safety and cost are the important concerns for the selection of oxidant, catalyst, and operating conditions for oxidative-extraction desulfurization processing. Different oxidants and operating conditions will result in different degrees of thiophene conversion, different product yields, operating costs and safety concerns. Considering air oxidation, for example, there are concerns that the reactivity and selectivity may not be adequate in the presence of hydrocarbons, and that the presence of nitrogen would require costly product recovery measures. However, using enriched air instead may seriously compromise process-operating safety. Several oxidants meet the required selectivity and safety criteria. Among them are three industrially viable oxidants, hydrogen peroxide, peroxyacetic acid and Caro""s acid.
Oxidation and solvent extraction of the target thiophene compounds has been explored by a number of companies over the past 50 years, [reference e. g., extraction processes by UOP U.S. Pat. No. 5,582,714 and GSK U.S. Pat. No. 5,494,572, and oxidation processes by Exxon RandEC U.S. Pat. No. 5,910,440, Novetech U.S. Pat. No. 5,824,207, Petro Star Inc Bonde, S. E. et al., ACS Div. Pet.Chem Prepritns 44(2), 199(1998), Fukuoka-ken U.S. Pat. No. 5,753,102, Ford et al U.S. Pat. No. 3,341,448, and Noble et al U.S. Pat. No. 2,749,284].
The oxidation reaction with substituted benzothiophenes proceeds to the corresponding sulfones at reasonable rates based on a number of reagents explored in the chemical literature [reference e. g., Bonde, S. E., Gore, W., and Dolbear, G. E., Am. Chem. Soc., Div. Petrol. Chem,. PREPRINTS, 44(2), (1999); Attar, A., Corcoran, W. H., IandEC Prod. Res. Dev, 17(2) 102 (1978); Zannikos, F., Lois, E., Stournas, S., Fuel Processing Technology, 42, 33 (1995); Guth, E. D., U.S. Pat. No. 3,847,800 (1975); Guth, E. D., U.S. Pat. No. 3,919,402 (1975); Tam, P. S., Kittrell, J. R., and Eldridge, J. W., IandEC Research, 29, 321-324 (1990)].
Other patents of interest include U.S. Pat. Nos. 3,413,307, 4,493,765, 4,954,229, 5,228,978, and 5,458,752.
The solvent extraction of the thiophene-oxides produced in the oxidation reaction becomes the second process step in the process. The need for alternative desulfurization processes for liquid fuels will increase dramatically with the implementation of ultra-low sulfur specification rules worldwide. As liquid fuel specifications drop below 100 ppm sulfur to 30 ppm or lower, particularly the small and medium size petroleum refiner must find alternative, cost effective process solutions that will allow the operation to remain competitive. Of course, the feed and final product specification will influence the process design directly.
Hydrocarbon fuels suitable for treatment with this process include atmospheric and vacuum gas-oils and products made from them. These include diesel fuel, home heating fuel, turbine fuels, kerosene, and various solvents and specialty fuels having similar distillation ranges. Hydrotreated middle distillates may also be treated with the process. The process may also be used for petroleum-derived liquid fuels boiling outside this temperature range; including gasoline range naphthas and various higher boiling gas oils and fuels.
The first objective of the process is to remove sulfur-aromatic compounds, i.e., substituted benzo- and dibenzo-thiophenes and their homologues that are costly and difficult to remove by hydroprocessing. A second objective is to allow simultaneous extraction of nitrogen-containing and aromatic hydrocarbons from the raffinate so that a desired combination of residual aromatics and low sulfur and nitrogen content can be obtained.
The process consists of a combination of several consecutive steps. These process steps are thiophene extraction; thiophene oxidation; thiophene-oxide and -dioxide extraction; raffinate recovery and polishing; solvent recovery; recycle solvent purification; and sulfur removal from the aromatic extract. The operating conditions are relatively mild throughout in the process. Pressures are near ambient and temperatures are less than 145xc2x0 C. throughout the process. The only chemical consumed in the process is hydrogen peroxide.
In the thiophene extraction step, the objective is to remove 5-65% of the thiophenic material, a substantial part of any present nitrogen-containing compounds, and parts of the aromatics from the feed stream. The feed is contacted in countercurrent flow with a solvent to yield a raffinate phase and an extract phase. The operating conditions during this phase are a temperature of between about 20 and 90xc2x0 C., pressures of between about 1 and 10 Bar. The solvent to feed ratio is between about 0.5:1 and 2:1.
The purpose of the thiophene oxidation step is to convert the remaining unextracted benzo- and dibenzo-thiophenes and their substituted homologues into the corresponding thiophene mono- and di-oxides in order to facilitate their subsequent extraction. Any nitrogen-containing compounds remaining in the treated liquid are converted to the corresponding N-oxide compounds. In this step, the raffinate from the thiophene extraction step above is mixed with an oxidant prepared in situ or previously formed. The feed is heated to the desired reaction temperature in a heat exchanger, and the reaction can be conducted either isothermally or adiabatically. Generally the oxidation operating conditions include a molar ratio of H2O2 to S between about 1:1 and 2.2:1, acetic acid content between about 5 and 45% of feed, solvent content between about 10 and 25% of feed, a temperature of between about 0 and 110 C., and a catalyst volume of less than about 5000 ppm sulfuric acid and preferably less than 1000 ppm. Once the reaction is complete, the effluent from the reactor flows directly to a thermal xe2x80x9cperoxide-eliminationxe2x80x9d unit comprising a feed-effluent heat exchanger, a heater providing 1-5 minutes residence time at 130-145xc2x0 C. (to eliminate all residual peroxides), and a product cooler. Due to the small residual amount of peroxides, the heat release is practically negligible.
The purpose of the thiophene-oxide and -dioxide extraction step of the process is to remove by extraction more than 90% of the various substituted benzo- and di-benzo-thiophene-oxides, including thiophene oxide and thiophene dioxides, also called thiophene sulfoxides and thiophene sulfones, and their various alkylated and arylated homologues. The process is also designed to remove any N-oxide compounds present in the oxidized liquids, as well as to remove a fraction of the aromatics from the feed stream. The effluent from the oxidation product cooler from the process above is contacted in countercurrent flow with the solvent to yield a raffinate phase and an extract phase. The extracting solvent is aqueous acetic acid with one or more co-solvents. The cosolvent may be selected from a family of acids including formic, acetic, propionic, butyric, isobutyric, valeric and various branched isomers, and caproic and its various branched isomers. The extraction column can be operated over a range of temperatures, solvent compositions and feed to solvent ratios to accommodate various feed compositions and raffinate product specifications for sulfur and aromatics. Typically, the operating conditions are temperatures between about 20 and 90xc2x0 C., pressures between about 1 and 10 Bar, and solvent to feed ratios between about 0.5:1 and 2.5:1. Other values outside these ranges are also possible. The extraction device can be a packed or multi-tray column with or without induced pulsation or intermittent mixing; however, any suitable combination of single or multi-stage liquid-liquid contacting and separation equipment can be used.
Depending on the solubility of the selected solvent and operating conditions, a smaller or larger amount of solvent remains in the raffinate effluent from the extraction. This solvent can be removed in various ways, including a combination of distillation, countercurrent water wash, and adsorption. In one embodiment of the process, the raffinate is washed in a single or multi-stage mixer-separator with water at between about 20 and 40 C. and ratios of water to raffinate between about 0.05:1 and 0.5:1. The bottom effluent, containing solvent, water and a small amount of oxidation catalyst, goes to the solvent recovery step. If required by the raffinate product specifications, the raffinate water wash is followed by a drying step using either a flash distillation or solid adsorbent bed such as silica, zeolite or alumina. This process step also eliminates any residual solvent remaining in the raffinate stream. As desired, this can be followed by a second adsorbent bed with activated granulated carbon, alumina, zeolite, fuller""s-earth or similar material to further reduce the residual sulfur compounds to meet or exceed the final product specification. Typically, a decrease of sulfur content between about 10 and 500 ppm S can be achieved in this process step. The election to remove more or less sulfur compounds in the extraction and adsorption sections is an economic decision that depends on the relative cost of the two operations.
Next, the extracts from the thiophene extraction and thiophene-oxide extraction can be processed singly or together depending on the specifications of the extract products. The solvent can be removed by a combination of distillations and water washes. After solvent recovery, depending on the character of the feedstock, the recovered extract typically consists of approximately 10-25 wt % sulfur-containing compounds and 10-30% aliphatic compounds, with the balance being aromatic compounds.
The solvent-containing streams from the water washes and co-solvent distillations are combined and fed to a solvent purification distillation column. The mixture of co-solvent and water is removed overhead and recycled to the water wash process steps, except for an amount corresponding to the water produced in the oxidation reaction, which is discharged. If desired for environmental reasons, the wastewater may pass through an activated carbon or similar absorber prior to discharge. The distillation column can be designed and operated so that the solvent recovered at the bottom meets recycle solvent specifications, i.e., it does not need to be pure solvent but may contain small amounts of hydrocarbons and co-solvents. In most cases where the feedstock composition results in a build-up of recycle hydrocarbons, a small side stream taken from the recycle solvent stream will resolve the problem.
The extract obtained from the thiophene and thiophene-oxide extraction can be further processed, separately or together, after solvent recovery. If desired the sulfur-containing compounds can be separated from the hydrocarbons for use as intermediate chemicals. Alternatively, the extract may be processed to remove the sulfur moiety to produce a low-sulfur fuel stream or aromatics feedstock. Several chemical and biochemical processes have the capability to accomplish these transformations.
The process design can be modified to accommodate a variety of hydrocarbon feeds; however, the boiling range of the feed will to a large extent determine the suitability of any specific solvent combination because of the need to recover the solvent for recycle. Several process design variations and economic optimizations are readily apparent to the process designer skilled in the art. For example, depending on the final product specifications and feedstock quality the first thiophene extraction process step may be designed to remove a smaller or larger amount of thiophenes and aromatic compounds, leaving the rest to be oxidized and extracted downstream. The design optimization is a trade-off between the cost of oxidation and the cost of the two extractions, including the cost of solvent recovery and recycle. The thiophene extraction therefore may be eliminated in some cases where the feedstock and product specifications so indicate.
It is an object of this invention to produce a method of extracting sulfur from hydrocarbons using acetic acid as a solvent.
It is another object of this invention to produce a process for extracting sulfur from hydrocarbons that has a first step of extracting substituted benzo- and di-benzo-thiophene compounds from the hydrocarbons.
It is another object of this invention to produce a process for extracting nitrogen-containing compounds from hydrocarbons.
It is yet another object of this invention to produce a process for extracting aromatic compounds from petroleum liquids and thereby to increase the cetane number in a diesel fuel.
It is yet another object of this invention to produce a process for extracting sulfur from hydrocarbons that has a thiophene oxidation step in the process.
It is yet another object of this invention to produce a process for extracting sulfur from hydrocarbons that has a thiophene-oxide and -dioxide extraction step in the process.
It is a further object of this invention to produce a process for extracting sulfur from hydrocarbons that has a raffinate recovery and polishing step.
It is yet another object of this invention to produce a process for extracting sulfur from hydrocarbons that has a solvent recovery step.
It is yet another object of this invention to produce a process for extracting sulfur from hydrocarbons that has a recycle solvent purification step.
It is a further object of this invention to produce a process for extracting sulfur from hydrocarbons that has a sulfur removal from the aromatic extract step.